1. Field of the Invention
The present invention relates to a method and an apparatus for aiding image interpretation, and to a computer-readable recording medium storing a program therefor. More specifically, the present invention relates to a method and an apparatus for aiding comparative image reading between two projection images by aligning projection images generated from two three-dimensional images representing the same subject photographed at different times, and to a computer-readable recording medium storing a program therefor.
2. Description of the Related Art
Inspection of a subject has been carried out in various fields, based on comparison of a difference between two or more images thereof. For example, in the field of manufacturing, an image of a product photographed at the time of production thereof is compared with an image thereof obtained after endurance tests thereon. A part showing a substantial difference is then detected as a part that needs to be improved in terms of endurance. In the field of medicine, a physician carries out comparative image reading on radiographs of a patient obtained at different times, and a plan of treatment can be discussed by understanding an interval change such as a progress or cure of a disease.
As a method of aiding such comparative image reading, a temporal subtraction method has been known. In a temporal subtraction method, an interval change is extracted and emphasized by generating a temporal subtraction image (hereinafter simply referred to as a subtraction image) through alignment of the same subject in images photographed at different times, with reference to a structural characteristic therein. More specifically, overall alignment processing (called global matching) is carried out for detecting an overall shift vector (a displacement vector) between two images. Thereafter, a shift vector for each of local regions in the images is detected and used for local alignment processing (called local matching) for determination of a shift vector for each of pixels. The subject in one of the two images is then aligned to the subject in the other image through non-linear transformation (called warping) on one of the images according to the shift vector. A subtraction image is then generated by subtraction processing on the warped image and the other image, which enables comparatively preferable alignment between the two images (see U.S. Pat. Nos. 5,359,513 and 5,790,690 and U.S. Patent Laid-Open Nos. 20010002934 and 20010048757 and “Digital image subtraction of temporally sequential chest images for detection of interval change”, by A. Kano, K. Doi, H. MacMahon, D. Hassell, M. L. Giger, Medical Physics, AAPM, Vol. 21, Issue 3, March 1994, p. 453-461).
In the inventions described in U.S. Pat. Nos. 5,359,513 and 5,790,690 and U.S. Patent Laid-Open Nos. 20010002934 and 20010048757 and in the paper written by A. Kano et al., a temporal subtraction method is applied to simple X-ray images wherein a whole thickness of a subject viewed from a direction of photography is projected. However, application of a temporal subtraction method has also been proposed for cross-sectional CT images along axial planes (planes perpendicular to the axis of a human body), in addition to simple X-ray images (see U.S. Pat. No. 6,363,163, for example).
However, in the invention described in U.S. Pat. No. 6,363,163, temporal subtraction is carried out on cross-sectional images having a thickness substantially equal to the thickness of slices used at the time of photography. Therefore, an interval change to be observed does not necessarily exist in the cross-sectional images to be processed. Consequently, comparative image reading is necessary between the cross-sectional images of the respective slices having been subjected to the temporal subtraction, and a person who carries out diagnosis (hereinafter referred to as a diagnostician) is burdened with the comparative image reading, which is a time-consuming process. Furthermore, in the invention described in U.S. Pat. No. 6,363,163, the thickness of the cross-sectional images is 10 mm. However, based on a progress in a photography technique such as multi-slice CT and necessity of high resolution for direct image reading between cross-sectional images, photography using a thickness of several millimeters is preferred, which makes the above-described problem more apparent.
For this reason, application of a temporal subtraction method has been proposed to projection images wherein a whole thickness of a subject viewed from a direction of observation is projected, based on a three-dimensional chest CT image (see U.S. Patent Laid-Open No. 20040114790, for example). According to this method, an interval change of a subject to be observed can be detected by viewing only one subtraction image, and a burden on a diagnostician is expected to be reduced greatly.
However, in the invention described in U.S. Patent Laid-Open No. 20040114790, if a change in three-dimensional positioning of the body of a subject (such as forward or backward inclination or rotation of the body) or a state of the subject (such as different respiratory states in the case of photography of chest images) occurs between images to be subjected to comparative image reading, the change causes the same structural characteristic to be projected in different shapes. Therefore, accuracy of alignment is lowered, and artifacts are generated in a subtraction image. For example, in the case where a three-dimensional structure as a subject having an elongated elliptic shape stands up in a vertical direction at the time of photography of an image in FIG. 3A while the subject stands up with slight inclination at the time of photography of another image shown in FIG. 3B, if all pixels in the direction of thickness of the subject are projected, the subject is projected as a circular object for the image in FIG. 3A while the subject is projected as an elliptic object for the image in FIG. 3B. For this reason, the structural characteristic used as reference for alignment does not correlate well between the two images, leading to a decrease in alignment accuracy.